In fiber optic transmission systems, signals are transmitted along optical fibers by optical frequency waves (light) generated by sources such as light emitting diode (LED) units, lasers, and the like. Optical fibers typically are fabricated of glass materials and, as optical fiber circuitry has developed, it has become necessary to provide connecting devices which can couple one optical fiber to another. It is important that the connection be in an end-to-end aligned relationship.
A traditional procedure for making a connection between ends of optical fibers is to initially remove a protective jacket from a given length of fiber at the end of the fiber to be joined. After the jacket is removed, a 250 micron (outside diameter) buffer is exposed which then can be stripped to expose a 125 micron (outer diameter) fiber. In the prior art, the fiber body then is threaded through a passage in a ferrule where it is affixed in place by adhesive and/or crimping. The fiber is inserted so as to extend well beyond a front surface of the ferrule. The exposed fiber material then is cleaved and polished. Any remaining adhesive is removed. The ferrules then are assembled into a connector assembly which is intended to position the optical fibers with their optical axes in alignment for connection to the fibers of a mating connector or other appropriate connecting device.
Fiber optic ribbon cable has become increasingly popular to provide multiple channels in a single cable structure. An optical ribbon cable is similar to any other well-known ribbon electrical cable to the extent that a plurality of optical fibers or channels are disposed in a line or a generally coplanar relationship. With these approaches, prior art practice for terminating the optical fibers of a fiber optic ribbon cable is generally similar to the procedure summarized above. In general, the unitary protective jacket surrounding the line of fibers is removed so that the buffered fibers are exposed which then are stripped such that the unprotected fibers project from the flat cable in a line. Typically, in the prior art these individual fibers must be inserted into respective individual holes or passages in a prefabricated connector ferrule. The passages align the fibers at a predetermined spacing for coupling to the ends of the fibers in a complementary connector ferrule or other connecting device.
This terminating process of the individual fibers of a multi-fiber cable is accompanied by a number of problems. Because of the very thin size and extremely fragile nature of the fibers, it can be tedious to insert a fiber into a single aligning hole or passage. Where a plurality of such fibers from a single cable need to be inserted into a plurality of passages, the difficulty is multiplied considerably. For example, if a single fiber of a multiple-fiber cable is broken, the stripped cable end and ferrule either must be discarded, reworked, or both. Since these processes typically have been carried out by hand, they can be extremely inefficient and result in unnecessary expense.
In the prior art, placing individual fibers of a multi-fiber cable into individual holes or passages in a connector ferrule results in a high percentage of rejects. The ferrules must be inspected hole by hole. In addition to fibers being broken, the holes-themselves may be too large or too small, or not circular, or have some other defect. Connector ferrules comprise bodies which are crystalline in nature, typically of ceramic material. Instead, they can be molded of a plastic or polymeric material. For multiple channel ferrules, the fiber-receiving holes or passages must be formed precisely to maintain a proper form or alignment and spacing between the fibers in order to prevent tolerance problems causing transmission losses during mating.
Alignment problems and tolerance problems such as those noted above are further complicated in connector assemblies wherein a pair of mating connector ferrules themselves are placed into mating condition by two alignment pins. These alignment pins typically have one end of each pin extending into a passage of the connector ferrule, and the opposite end of the pin is inserted into a passage in the mating connector ferrule, with a chamfered lead-in on the pin for alignment. The problems of maintaining precise tolerances with the alignment pins and their passages must be added to the tolerance problems in maintaining precise spacing and alignment of the individual holes for the optical fibers of the fiber optic cable. It can be understood why there are such a high number of rejects during the application of prior art connector units.
With further reference to DWDM products, multiplexing can be used to combine channels of different wavelengths, whereas at the receiving end demultiplexing separates the channels from one another with a minimum inter-channel cross talk. In DWDM products, the separation between adjacent devices is designed to be fairly narrow in order to increase device capacity. A typical separation is 200 GHz to 50 GHz, corresponding to 1.6 nm and 0.4 nm in wavelength, respectively. Currently available DWDM products are of the arrayed waveguide (AWG) type, such as of the 1×8 (1 input, 8 outputs) 1×16, 1×32 and 1×64 configurations. It will be appreciated that a small difference among the lengths of the output waveguides is responsible for separating the stream of wavelengths from one another.
One of the most important functions in connection with DWDM products is attaching fibers for coupling light in and out of the device with minimum loss. In the past, this has required input and output fibers being first attached to separate platforms at appropriate distances using adhesive glue or curable epoxy. In this prior art approach, these platforms then are brought in close proximity with a device such as a multiplexing and/or demultiplexing device and actively aligned to the appropriate waveguides. An example of a prior art approach is found in Yamane et al. U.S. Pat. No. 5,557,695, in which so-called integral waveguides are provided and the optical fibers are laid in guide grooves as part of the connection procedure.
In the prior art active alignment practice, light is launched into the input fibers, and light emanating from the output fibers is monitored. Determining the optimum coupling position requires using x-y-z movement and rotational movement of the device and the platforms with respect to each other in the vertical and horizontal axes. The pieces then are locked in place with adhesive, glue or curable epoxy. From this it will be appreciated that active alignment is tedious, involved, expensive and slow. Using a fiber optic connector ferrule is useful in precisely aligning a line of fibers for alignment with a complementary ferrule. An example of such an approach and of a type of fixture for assembling same is shown in Bunin et al. U.S. Pat. No. 5,907,657, incorporated hereinto by reference. While ferrules of this type are an important advance in the art, further improvements are realized according to the present invention which achieves an advantageously passive alignment requiring no light up or monitoring of light in the fibers. The passive alignment process of the invention is fast, reproducible, easy and cost effective. So advanced is this approach that accurate alignment according to the invention is achievable in the field by straightforward component removal and replacement. This is a marked improvement over prior art approaches which require alignment in a laboratory environment, typically requiring very expensive alignment equipment.